The reliability of small electronic components depends to a great extent upon how well the component is hermetically sealed from reactive gases and water vapor. The test for determining the adequacy of the hermetic seal is important.
Presently, there are several methods for determining the leak rate in small electronic components. Several methods require that the electronic component be exposed or "bombed" with a gas. Usually, the gas is helium or sometimes argon or krypton. After exposing the electronic component for a specified time and pressure to the gas, the component is removed and tested for leaks. Breaks or defects in the hermetic seal are revealed by the gas which has infiltrated the component and which is detected as a "leak" when the gas flows out of a break or defect in the hermetic seal. Most tests are designed either for gross or large leaks or for fine or small leaks. The "bubble" test is frequently used as a gross leak test. In the bubble test, the device after having been exposed to the gas is submerged in a suitable liquid and visually inspected for the formation of bubbles indicative of a leak. See Myers U.S. Pat. No. 3,646,804 and Farrell U.S. Pat. No. 3,738,158. The bubble test has the obvious disadvantage of inadequate quantitative results and inability to detect small leaks.
Other gross leak test methods have also been used. One such method involves exposing a small electronic component to a gas and measuring the weight gain. Another involves exposing the component to a hydrocarbon gas and measuring the hydrocarbon vapor released while pressurizing the component with another gas.
Finally, leak tests on electronic components have employed mass spectrometers. One such device is illustrated in Altshuler U.S. Pat. No. 3,578,758. Although helium leak detectors have been used for fine leak testing small electronic components, one disadvantage is that present helium leak detectors are not totally suited for detecting larger leaks. If a component having a large leak is subjected to a helium leak detector, all the helium may be exhausted out of the component by the vacuum system before detection measurement. In any event, such a helium leak detector reduces the apparent size of a large leak. In general, present helium leak detectors for small electronic components have inadequate sensitivity dynamic range.
Another test which has been employed for detecting leaks in electronic components employes radioactive krypton. The components are exposed to radioactive krypton and then tests are made to determine if any radioactive emissions can be detected.
Getter pumps which have been employed in ultra sensitive leak detectors (e.g., Bergquist U.S. Pat. No. 4,492,110) have not been employed in leak detectors for small components since getters are unable to quickly handle the relatively large volume of purge carrier gas which must be employed in helium leak detecting small components.
Cryogenic pumps have been previously employed in creating vacuums. However, cryogenic pumps have previously not been employed as roughing pumps for leak detector systems. One deficiency of a typical cryogenic pump is that it is unable to handle a relatively large volume of purge carrier gas used in leak detecting small components. Adsorption of a large volume of gas warms up the cryogenic pump which results in desorption of the purge carrier gas. That in turn raises the pressure to an unacceptable level for the mass spectrometer. Secondly, cryogenic pumps adsorb helium and may later desorb helium which destroys the efficacy of the detection of helium by the mass spectrometer.